Share and manage one singleton scripting container.

As this is a very heavy weight component, but the
gateway itself is not thread safe (according to Kristian)
Added javadoc to remind us about this fact
Merge remote-tracking branch 'origin/NEXUS-6905-merge-rubygems' into NEXUS-6905-merge-rubygems-managed-ruby-container
Using plain container
Merge pull request #777 from sonatype/NEXUS-6905-merge-rubygems-managed-ruby-container

[Ruby] Share and manage one singleton scripting container.